The present invention relates to a microcomputer-controlled system for measuring the volume or quantity of liquid in one or more tanks. Although the present invention has application in a variety of liquid gaging systems, it will be described in the context of an aircraft fuel gaging system.
In the aircraft industry, a basic sensor for measuring fuel volume or quantity has long been the capacitance sensor which has been accepted for many years as a rugged, reliable device. In the above mentioned co-pending application filed on May 4, 1979 (application Ser. No. 036,119) now U.S. Pat. No. 4,258,422, a system was described which provided significant improvement in the sensor accuracy of liquid gaging systems, including systems for measuring aircraft fuel volume or quantity with capacitive sensors. As with the present system, the system described in application Ser. No. 036,119 achieves improved sensor gaging accuracy and flexibility by use of a microcomputer or similar device to provide tank shape and volume, tank or aircraft attitude, and similar characterization which in the prior art was formerly only approximated by means of physically characterized (shaped) fuel gage probes.
In this manner, like the liquid gaging system described in application Ser. No. 036,119, now U.S. Pat. No. 4,258,422 the present system provides a number of significant advantages over conventional liquid gaging systems. These advantages include the need for a fewer number of sensors or probes in each tank, simplified probe construction by elimination of physical characterization, improved system accuracy by characterizing for tank geometry and tank or airplane attitude in a microcomputer or other digital system, reduced system weight by decreasing the number of probes, and simplified installation for the aircraft manufacturer by requiring fewer probes. Digital characterization also provides a more flexible design which can accommodate tank changes with minor hardware impacts.
The present system was developed to provide a liquid gaging system having even greater accuracy and flexibility than previously disclosed systems. One feature of the present system providing significant additional accuracy and flexibility is the individual measurement of the wetted length of each probe. This is in contrast to typical prior art systems and the system described in application Ser. No. 036,119 wherein the total wetted length of all probes in a particular tank is measured and used for determination of liquid volume or quantity. Thus, in the system described in application Ser. No. 036,119, a wetted length signal related to the portion of all probes in a tank wetted by the liquid is used in conjunction with data stored in attitude tables for determining liquid volume or quantity in a tank.
In contrast, one advantage of the present system is that no attitude sensor is required, and no signal need be received by the system to separately indicate the attitude of the tank or aircraft. Elimination of an attitude sensor is made possible in part through the previously mentioned individual monitoring of the wetted length of each probe. Knowing the individual wetted length of each probe, the determination of fuel volume or quantity can be made through stored data having the effects of attitude, tank geometry, the number and location of the probes, and similar factors already included. Thus, the present system provides the basis for significant simplification over systems requiring attitude sensors since attitude sensors with sufficient precision and reliability are complex and expensive. Accordingly, the present system provides the basis for improving reliability and reducing maintenance requirements.
The present system also provides significantly increased accuracy over the accuracy of systems available in the prior art. In the preferred embodiment, each probe in the present system is divided into theoretical sections having a length, and at least one characterization table corresponding to each probe is used with data related to the theoretical sections and the wetted length of the probe to directly determine fuel volume or quantity. By having a plurality of characterization tables corresponding to each probe, multiple characterizations governing various conditions can be used. Thus, for example, there can be separate characterizations for ground and flight conditions and for separate ranges of liquid volume or quantity. As will be further discussed in this application, such multiple characterization of each probe can significantly increase system accuracy.
In addition, prior art systems compatible with individual monitoring of physically characterized probes can be retrofitted with the present system in order to provide multiple characterization of each probe, thereby significantly increasing the accuracy of the such systems.
The present system also incorporates digital null balance circuitry and a rebalance approximation sequence which provides a high-speed reading means for very quickly and accurately determining the capacitance of each sensor as the sensors are individually monitored. The digital null balance circuitry in the present system is in contrast to relatively slow prior art circuitry using means such as up-down counters to monitor a reading. A high-speed accurate system is of substantial advantage since, for example, in an aircraft fuel gaging system, a relatively short probe could very quickly go from being 100 percent wetted to being completely unwetted. In such a situation, the relatively slow prior art reading means would be incompatible with multiplexed probe readings such as those in the present system.
Another advantage of the present system is its ability to monitor or fault isolate various components for malfunction. For example, through a contamination monitor and related circuitry, each probe and dielectric sensor may be monitored for two levels of contamination, a first level indicating that the component is becoming contaminated but is still useable and a second level indicating the component is no longer useful.
Another example of the fault isolation capabilities of the present system relates to two precision reference components, a reference resistor and a reference capacitor, each of which is driven by the system excitation signal generator. By monitoring the current through the reference resistor using the null balance circuitry, a primary check of the excitation signal level is obtained, and a secondary check of the null detecting circuitry is made. By monitoring the excitation signal through the reference resistor via the contamination monitor, a primary check of the contamination monitor is made, and a secondary check of the excitation signal level is obtained. By monitoring the current through the reference capacitor, a primary check of the excitation signal frequency is obtained, and a secondary check of the null detecting circuitry is made.
A still further advantage of the present system relates to elimination of a significant portion of the additional error introduced by complete loss of a probe such as when a single capacitive probe is disabled and provides a zero capacitance reading or when a probe is determined unusable by the contamination monitor. In a preferred embodiment of the present system, one or more sister probes are identified for each probe and used to estimate a failed probe's wetted sensing length. In this manner, if a probe does malfunction completely, substantially less precision is lost than is typical prior art systems under such circumstances.
An even further advantage of the present system is that, since individual probes are monitored for wetted sensing length, individual capacitive probes can be monitored while unwetted in order to determine their true unwetted capacitance, including any stray capacitance introduced during installation. Combined with a storage device such as an electrically programmable read-only memory (EPROM), monitoring individual capacitive probes while unwetted allows exact empty capacitances to be stored in the microcomputer for use in making extremely precise liquid volume or quantity determinations. The feature is particularly applicable upon installation of a system or after an overhaul during which additional or different stray capacitances may have been introduced.